1. Technical Field
The present invention is in the field of oligonucleotide compounds of biological activity. More particularly, the present invention is in the field of oligonucleotide compounds which include N-methyl thiolated heterocyclic bases and have antiviral activity.
2. Brief Description of the Prior Art
Nucleosides which contain other than the major naturally occurring heterocyclic bases and related derivatives have been used in the prior art as antiviral agents, for example for treatment of herpes simplex infections. In the present state-of-the-art virtually all drugs utilized for treatment of infection of humans by the human immunodeficiency virus (HIV) are also nucleosides or nucleosides analogs, with 3'-azido-3'-deoxythymidine (AZT) and 2',3'-dideoxycytidine (DDC) serving as examples.
The HIV virus is a retrovirus which carries the virus genom in RNA rather than DNA form. Replication of the HIV virus requires the utilization of a reverse transcriptase enzyme which, in essence, makes a DNA copy of the virus genome which is then incorporated into the DNA of the infected human cells. Several of the therapeutic agents or their metabolic products such as the AZT triphosphate have been shown to be inhibitors of the HIV reverse transcriptase enzyme. The drugs currently used for treatment of HIV infections in humans, are however quite toxic. As is well known, while the state-of-the-art drugs provide some benefit, they are incapable of curing or definitely halting the progress of an advanced stage HIV infection in humans. It is well known, that more effective chemotherapeutic agents are needed for the treatment of HIV infections in humans.
Human cytomegalovirus is a DNA virus which primarily causes respiratory problems and blindness. Ganciclovir (DHPG) is a drug which is used in the state-of-the-art for treatment of humans infected with this virus. It is still considered desirable in the art to develop further chemotherapeutic agents for treatment of by human cytomegalovirus infections.
Certain thiolated purines, pyrimidines and related heterocyclic, bases and the corresponding nucleosides (such as ribosides and deoxyribosides) have been known in the art for a long time. Generally speaking, in these compounds an oxo function (O) of a naturally occuring heterocycle is replaced by a thio (S) group. 6-Thiopurine (also known as 6-mercaptopurine), 6-thio-2-aminopurine ("thioguanine"), 4-thio-2-oxopyrimidine ("thiouracil") and 4-thio-2-oxo-5-methylpyrimidine ("thiothymidine) serve as examples. 6-Thiopurine is a potent inhibitor of an enzyme involved in the biosynthesis of the purine bases which are incorporated into nucleic acids, and although it has been used as drug in the chemotherapy of cancer, it is considered quite toxic. N-methyl derivatives of the above-noted thiolated purines and pyrimidines and their nucleosides have also been known in the prior art, with 1-N-methyl-6-thiopurine serving as an example. (See Broom A. D. et al. J. Heterocyclic Chem. 12, 171-174 (1975)).
Relatively recently, polyribonucleotides of 1-N-methyl-6-thioinosinic acid, of 1-N-methyl-6-thioguanylic acid and of 6-thioguanylic acid (the latter complexed with polycytidylic acid) having molecular weight of 10.sup.5 to 10.sup.6 daltons were discovered in the prior art to be agents capable of inhibiting growth of HIV in certain human cell lines. The polyribonucleotides of 1-N-methyl-6-thioinosinic acid is described by E. W. Chan et al. in J. Gen. Virol. 1981, 52, 291-299. The polyribonucleotide of 1-N-methyl-6-thioguanylic acid is disclosed by V. Amarnath et al. in Biochim. Biophys. Acta 1977, 479, 16-23. The complex of poly-6-thioguanylic acid with polycytidylic acid as an antitumor agent is described by V. Amarnath et al. in Biochemistry 1976 15, 4386-4389.
Certain oligo[(2'-O-methyl)ribonucleoside phosphorothioates] were reported as weak inhibitors of the replication of the HIV virus. (Saamu Shibahara et al. Nucleic Acid Research, Volume 17, Number 1, pp239-252 (1989)).